In general, conventional far-field millimeter wave imaging systems have been widely used in various applications such as security screening (e.g., concealed weapon detection), collision avoidance radars, and for safe landing in poor-visibility conditions. These conventional systems are usually very expensive, complex, and bulky. For example, one known conventional imaging system is based on a complex passive millimeter wave video camera having 1024 receiver modules operating at 89 GHz. In this system, an 18-inch diameter plastic lens is used to collect and focus radiation yielding a diffraction-limited 0.5° angular resolution.
Although the image quality of these systems is impressive, due to the complexity of these far-field imagers and their cost, they have not been used in many high-volume applications such as medical imaging. In addition to their high cost, the resolution achieved by these imagers is not high enough to be used in medical applications, where a resolution of 1 mm or less is required. These systems perform far-field imaging where the highest image resolution that can be achieved is set by the diffraction limit. For example, a commercially available 18-inch 89 GHz camera has an angular resolution of 0.5° which is equivalent to 8.7 mm spatial resolution for an antenna-object distance of 1 m. Thus, two main drawbacks of current far-field imagers that prevent them from being used in medical applications are their high cost and low resolution set by the diffraction limit.